It is worth to consider the origin of the current warnings for bad https
sites. The premise was that if a user types https:// , then she explicitly
asked about security and all violations should be reported. Typing http://
was considered that user communicated that she did not care and no reports
should be generated. That premise naturally lead to the current situation
when a site with self-signed is reported while plain http:// is not.
What went wrong with warnings was that even users that were aware that for
their banking site they need to type https:, they most often than not
asumed that typing https:// by itself guaranteed that everything should be
fine. Any warnings about security were perceived as a problem with the
browser that should be ignored, not as a problem with the site or a
potential attack.
This interpretation of ssl warnings as brokenness of the browser is
enforced when a user follows links to a https site. A natural assumption is
that the links worked for the author of the document and any warnings means
that something was wrong with the browser.
In view of that it is important to make very clear for the user that any
warnings about plain HTTP indicates that the site operator has not done
their job properly. Nothing is wrong with the browser.
I suspect a message that puts a shame on the site for not implementing
https would be much better then informing the user that a connection can be
monitored or subverted.